The present invention relates to a ferromagnetic resonance measurement.
A few elements, notably iron, cobalt and nickel show the effect of spontaneous magnetisation, which is termed ferromagnetism. In a ferromagnetic material atoms effectively act as atomic bar magnets which interact co-operatively so that large groups of atoms within a structure have a common orientation of their magnetism. In a quantum mechanical description, the alignment of magnet moments is ascribed to the exchange interaction, which energetically favours magnetic order.
Ferromagnetic resonance measurements are conceptually similar to nuclear magnetic resonance measurements, which form the basis of magnetic resonance imaging (MRI) scanners. A ferromagnetic sample is located in a strong magnetic field. The effect of the strong magnetic field is to align the atomic magnetic moments in a single orientation and to alter the energy levels of excited states of the atoms. Microwave radiation at a predetermined frequency is directed at the sample. The strength of the magnetic field is increased gradually, thereby altering the degree of alignment of the atoms and modifying the energy levels of the excited states of the atoms. When an energy level of an excited state is equal to the energy of the incident microwave photons, the microwave radiation will be resonantly absorbed by the ferromagnetic material.
The amount of microwave radiation absorbed by the sample is monitored using a microwave detector. The values of magnetic field strength which give rise to absorption of the microwave radiation are indicative of the structure of the sample being tested.
In known ferromagnetic resonance measurements, a sample of a material to be tested is located within a resonant cavity, the resonance of the cavity being selected for the frequency of microwave radiation that is to be directed at the sample. The resonant cavity enhances the signal to noise ratio of the ferromagnetic resonance measurement.
Recently, a modified ferromagnetic resonance measurement has been developed wherein a single value, magnetic field is applied to a sample, and microwave radiation directed at the sample is swept over a range of frequencies (M. E. Unwin et al, A novel broadband ferromagnetic resonance spectrometer, Journal of Magnetism and Magnetic Materials, 205(1999) 199–208). The sample is located in a waveguide in such a way as to isolate the microwave radiation so that it travels through the sample in one direction only. This arrangement is based upon a resonant isolator, a microwave circuit device which allows microwave propagation in one direction only.
A disadvantage of known ferromagnetic resonance measurement methods is that they require a sample to be located in a resonant cavity or in a waveguide. Thus, a ferromagnetic resonance measurement of a large sample cannot be carried out, unless part of that sample is removed and located within a resonant cavity or waveguide.
It is an object of the present invention to provide a ferromagnetic resonance measurement method which overcomes or mitigates the above disadvantage.